The terms control and control systems generally refer to the control of a device, process or system by monitoring one or more of its characteristics. This is used to insure that output, processing, quality and/or efficiency remain within desired parameters over the course of time. In many control systems, digital data processing or other automated apparatus monitor a device, process or system and automatically adjust its operational parameters. In other control systems, such apparatus monitor the device, process or system and display alarms or other indicia of its characteristics, leaving responsibility for adjustment to the operator.
Control is used in a number of fields. Process control, for example, is typically employed in the manufacturing sector for process, repetitive and discrete manufactures, although, it also has wide application in utility and other service industries. Modern day control systems typically include a combination of field devices, control devices and controllers, the functions of which may overlap or be combined. Field devices include temperature, flow and other sensors that measure characteristics of the device, process or system being controlled. Control devices include valves, actuators, and the like, that control the device, process or system itself.
Controllers generate settings for the control devices based on measurements from the field devices. Controller operation is typically based on a control algorithm that maintains a control system at a desired level, or drives it to that level, by minimizing differences between the values measures by the sensors and, for example, the set-point defined by the operator. Controllers may be networked or otherwise connected to other computing apparatus that facilitate monitoring or administration. At the lowest level of the hierarchy are control modules that directly manipulate field devices. At a higher level, equipment modules coordinate a function's control modules as well as other equipment modules and may execute phases of the manufacturing process (such as setting controller constants and modes). Units, at a still higher level of the hierarchy, coordinate the functions of equipment and control modules. Process cells orchestrate all processing activities required to produce a manufacturing batch, scheduling, preparing and monitoring equipment or resources, and so forth.
The principal function of controllers is executing control algorithms for the real time monitoring and control of devices, processes or systems. They typically have neither the computing power nor user interfaces required to facilitate the design of a control algorithm. Instead, the art has developed general purpose computers (work stations) running software that permit an operator to graphically model a device, process or system and the desired strategy for controlling it. This includes enumerating field devices, control devices, controllers and other apparatus that will be used for control, specifying their interrelationships, and the information that will be transferred among them, as well as detailing the calculations and methodology they will apply for purposes of control. Once a model is complete and tested, the control algorithm is downloaded to the controllers.
Historically, the process control industry has used manual operations such as manually reading level and pressure gauges, turning valve wheels, and so forth, to operate the measurement and control field devices within a process. With the emergence of the microprocessor-based distributed control system (DCS), distributed electronic process control became prevalent in the process control industry. A DCS includes an analog or a digital computer, such as a programmable logic controller, connected to numerous electronic monitoring and control devices, for example, electronic sensors, transmitters, current-to-pressure transducers, valve positioners, and so forth, located throughout a process. The DCS computer stores and implements a centralized and, frequently, complex control scheme to effect measurement and control of devices within the process to thereby control process parameters according to some overall control scheme.
Usually, however, the control scheme implemented by a DCS is proprietary to the DCS controller manufacturer which, in turn, makes the DCS difficult and expensive to expand, upgrade, reprogram and service because the DCS provider must become involved in an integral way to perform any of these activities. Furthermore, the equipment that can be used by, or connected within, any particular DCS may be limited due to the proprietary nature of the DCS controller and the fact that a DCS controller provider may not support certain devices or functions of devices manufactured by other vendors.
The controllers communicate with the devices via input/output (I/O) devices. The I/O devices implement the communications protocol used in the process control network, and control the communications between the controllers and the devices on the segments. Although the I/O devices facilitate the communications between the controllers and the devices on the segments, process control ceases, at least with respect to devices on a particular segment, if the I/O device for the segment goes out of service for whatever reason.
The impact of a disabled I/O device and disruption to process control may be reduced by providing a backup I/O device that is connected to a segment that takes over for the disabled I/O device. However, the transition from the disabled I/O device to a backup I/O device is not without complications, and disruption in the process control still occurs. Currently known backup I/O devices are not updated with the current information stored in the primary I/O device, such as current values of process variables, functional software that may reside in the I/O device, the communication schedules for the devices on the segment, and so forth. Also, in some implementations, the backup I/O device does not automatically assume control when the primary I/O device becomes disabled, resulting in a delay in performing process control until a user activates the backup I/O device. In addition, in some protocols, the devices are configured to communicate specifically with the primary I/O device and must be reconfigured to communicate with the backup I/O device before the backup I/O device can take over communications on the segment.
A controller node is constructed by combining modules with selected functionality into one device. In general, a modular architecture allows the selection of hardware capabilities to strictly match the specific requirements of a particular application without compromising future expansion needs.
Current modular solutions usually require that one build a device by selecting a single, main processing module, a single power module, various optional expansion input/output (I/O) modules, and a single optional networking module. However, with this arrangement, the single processing module bears an increasing computational burden as modules are added, since the aggregate device cannot have multiple processing modules working in parallel. Thus, an unaddressed need exists in the industry to address the aforementioned efficiencies and inadequacies.